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Angiostrongyliasis

Parasite Biology

Causal Agents

The nematode (roundworm) Angiostrongylus cantonensis, the rat lungworm, is the most common cause of human eosinophilic meningitis. In addition, Angiostrongylus (Parastrongylus) costaricensis is the causal agent of abdominal, or intestinal, angiostrongyliasis.

Life Cycle

Adult worms of A. cantonensis live in the pulmonary arteries of rats. The females lay eggs that hatch, yielding first-stage larvae, in the terminal branches of the pulmonary arteries. The first-stage larvae migrate to the pharynx, are swallowed, and passed in the feces. They penetrate, or are ingested by, an intermediate host (snail or slug). After two molts, third-stage larvae are produced, which are infective to mammalian hosts. When the mollusk is ingested by the definitive host, the third-stage larvae migrate to the brain where they develop into young adults. The young adults return to the venous system and then the pulmonary arteries where they become sexually mature. Of note, various animals act as paratenic (transport) hosts: after ingesting the infected snails, they carry the third-stage larvae which can resume their development when the paratenic host is ingested by a definitive host. Humans can acquire the infection by eating raw or undercooked snails or slugs infected with the parasite; they may also acquire the infection by eating raw produce that contains a small snail or slug, or part of one. There is some question whether or not larvae can exit the infected mollusks in slime (which may be infective to humans if ingested, for example, on produce). The disease can also be acquired by ingestion of contaminated or infected paratenic animals (crabs, freshwater shrimps). In humans, juvenile worms migrate to the brain, or rarely in the lungs, where the worms ultimately die.

The life cycle of Angiostrongylus (Parastrongylus) costaricensis is similar, except that the adult worms reside in the arterioles of the ileocecal area of the definitive host. In humans, A. costaricensis often reaches sexual maturity and release eggs into the intestinal tissues. The eggs and larvae degenerate and cause intense local inflammatory reactions and do not appear to be shed in the stool.

Geographic Distribution

Most cases of eosinophilic meningitis have been reported from Southeast Asia and the Pacific Basin, although the infection is spreading to many other areas of the world, including Africa and the Caribbean. Abdominal angiostrongyliasis has been reported from Costa Rica, and occurs most commonly in young children.

Clinical Presentation

Clinical symptoms of eosinophilic meningitis are caused by the presence of larvae in the brain and by local host reactions. Symptoms include severe headaches, nausea, vomiting, neck stiffness, seizures, and neurologic abnormalities. Occasionally, ocular invasion occurs. Eosinophilia is present in most of cases. Most patients recover fully. Abdominal angiostrongyliasis mimics appendicitis, with eosinophilia.

Image Gallery

Angiostrongylus costaricensis eggs and larvae in tissue stained with hematoxylin and eosin (H&E).

In humans, Angiostrongylus eggs and larvae are not normally excreted but remain sequestered in tissues. A. costaricensis infections are predominantly abdominal; both eggs and larvae (occasionally adult worms) can be identified in biopsy or surgical specimens of intestinal tissue where the eggs and larvae are engulfed in giant cells and/or granulomas. A. cantonensis infections are predominantly cerebral, being one of the most common causes of eosinophilic meningitis, although developing but immature adult worms can on occasion migrate to the lungs. No A. cantonensis eggs or larvae have been recognized in human tissues.

The larvae of A. costaricensis need to be distinguished from larvae of Strongyloides stercoralis. A. costaricensis first stage (L1) larvae tend to be slightly smaller in diameter than Strongyloides and have a single lateral ala, whereas Strongyloides third stage (L3) larvae have minute double lateral alae. The alae can be difficult to discern in most histologic sections. However, the presence of granulomas containing thin shelled eggs and/or larvae generally serve to distinguish A. costaricensis infections from Strongyloides infections.

Figure A: Angiostrongylus sp. male worm, approximately 4.25 mm in length, recovered from vitreous humor of a human patient. The worm is most likely A. cantonensis based on the patient's geographic location.

Figure B: Angiostrongylus worm. Note the very long spicules (black arrows), one of the indications that this is a male worm.

Figure C: Higher magnification of the worm in Figure B.

Figure D: Angiostrongylus adult worm recovered from vitreous humor of a human patient. The bursa is one indication that this is a male worm. The worm is most likely A. cantonensis based on the patient's geographic location.

Figure E: Same specimen as in Figure D but in a different focal plane.

A. cantonensis larvae recovered from slugs.

Third stage (L3), infective larvae would not be found in human tissue; the images below are not clinically diagnostic. L3 larvae are infective to humans, who serve as incidental hosts.

Figure C: Higher magnification of Image B. Note the terminal projection on the tip of the tail which is characteristic of A. cantonensis.

A. cantonensis larvae recovered from slugs.

Figure A: Angiostrongylus cantonensis third stage (L3), infective larva, in a wet mount, recovered from a slug. Note the terminal projection on the tip of the tail which is characteristic of A. cantonensis.

Laboratory Diagnosis

Laboratory Diagnosis

Morphologic Diagnosis

In eosinophilic meningitis with A. cantonensis, the cerebrospinal fluid (CSF) is abnormal (elevated pressure, proteins, and leukocytes; eosinophilia). On rare occasions, larvae have been found in the CSF. In abdominal angiostrongyliasis with A. costaricensis, eggs and larvae can be identified in biopsy specimens.

Molecular Diagnosis

The CDC offers a real-time PCR assay for the specific detection of A. cantonensis in human CSF specimens. The CSF should be kept cold and shipped to CDC on ice pack as soon as possible; alternatively it can be kept frozen until shipment is arranged. Shipment on dry ice is optional. Recommended volume of CSF is at least 2 ml. Volumes less than 1 ml may lead to reduced test sensitivity.

No specific molecular tests are available for A. costaricensis. A conventional PCR followed by DNA sequencing can be performed for A. costaricensis on a case-by case basis. Acceptable specimen type is tissue biopsies.